How to make a watermark?

my friend made a watermark for me and she sent it to which had a white background. I removed it via aviary. but when i saved my work i could not open it. I want to use a batch software program to put my logo on all my pictures without the white paper background. I do not know what i am doing wrong because when i try to open the saved file it asks me what program do i want to open it with.. is there a way for me to get the logo only and not the white background?! please help i am so confused at this but i would like my pics to have my watermark. thanks in advance!